An ignition coil is known to be in practical use and takes, for example, the form of a so called rod-type ignition coil of a spark-ignition engine of a motor vehicle.
An ignition coil represents an energy-transmitting, high-voltage source used in a spark-ignition engine for controlling at least one spark plug, whose electrodes are inserted into a combustion chamber of the internal combustion engine, and which is used for igniting a fuel mixture introduced into the combustion chamber, whereby a movement of an engine piston and, therefore, of an engine crankshaft is initiated.
A conventional type of ignition coil has two concentric windings, which have different numbers of turns per unit length and are magnetically coupled to each other. The magnetic coupling is strengthened by a magnetically active coil core.
The coil winding having the lower number of turns per unit length is referred to as the primary winding and is assigned to a primary circuit. The coil winding having the larger number of turns per unit length is referred to as a secondary winding and is assigned to a secondary circuit, which also includes the spark plug in question. The primary winding represents an excitation winding and is supplied with energy by an electrical system of the motor vehicle in question. It is controlled by a control unit of the internal combustion engine, using an electronic switch that can be designed as an output stage or igniter. In order to produce, at the secondary winding connected to the spark plug, a high voltage that generates a spark between the electrodes of the spark plug, a deliberate break is made in the primary circuit.
In rod-type ignition coils, the secondary winding is normally the inner winding, while the primary winding constitutes the outer winding. One end of the secondary winding is connected to a high-voltage terminal of the spark plug. The other end of the secondary winding is connected to ground.
Because of the spark formation between its electrodes, the spark plug represents a source of interference with regard to electromagnetic compatibility (EMC) and may therefore impair the performance of other components of the system formed by the motor-vehicle electrical system. The interference may spread via both line-conduction and radiation, the line-conducted portion normally predominating. Therefore, electrical interference signals, which are produced by the spark formation initiated at the spark plug, may be transmitted from the spark plug into the vehicle electrical system, via the ignition coil. These interference signals couple over both inductively and capacitively from the secondary circuit of the ignition coil into the primary circuit and, in this manner, they enter the vehicle electrical system. The capacitive coupling occurs due to so-called parasitic capacitances between the primary winding and the secondary winding of the ignition coil. In known methods heretofore, e.g. anti-interference capacitors or the like are used to eliminate interference.
An object of the present invention is to provide, for an internal combustion engine, an ignition coil that includes a simple means for eliminating interference from the motor vehicle electrical system.